1. Field of the Invention
The present invention relates to zinc secondary batteries such as nickel-zinc secondary batteries, silver oxide-zinc secondary batteries, manganese oxide-zinc secondary batteries, zinc-air secondary batteries, and other alkaline zinc secondary batteries.
2. Description of the Related Art
Development and examination of nickel-zinc secondary batteries have been carried out for many years, but nickel-zinc secondary batteries are not yet in practical use. This is due to the problem in that zinc, which constitutes the negative electrode, generates tree-like crystals called dendrites during charging, and dendrites break through the separator, thus resulting in a short circuit with the positive electrode. On the other hand, nickel-cadmium batteries and nickel-metal hydride batteries are already commercially available. However, nickel-zinc secondary batteries are advantageous in that they have an extremely high theoretical capacity density about 5 times that of nickel-cadmium secondary batteries, 2.5 times that of nickel-metal hydride secondary batteries, and 1.3 times that of lithium-ion batteries, and also material prices of nickel-zinc secondary batteries are low. Therefore, a technique for preventing a short circuit caused by zinc dendrites in nickel-zinc secondary batteries is strongly desired.
Patent Document 1 (JP H6-196199A) proposes suppression of dendrites by a separator in a nickel-zinc battery. In this document, a microporous, multi-layered metal oxide layer provided on a resin separator allows ions that are involved in a battery reaction to penetrate the separator, but makes it difficult for the metal that has undergone dendritic growth to penetrate the separator. It is disclosed that the material of the multi-layer metal oxide is one or more selected from alumina, titanium oxide, silica, cerium oxide, zirconium oxide, magnesium oxide, chromium oxide, calcium oxide, tin oxide, indium oxide, and germanium oxide.
Patent Document 2 (JP H11-054103A) discloses enhancement of dendrite resistance by reducing the rate of dendrite growth by a porous membrane separator in which a partially-saponified crosslinked polyvinyl alcohol membrane is impregnated with glycerin.
Patent Document 3 (JP 2008-539559A) discloses a separator having a barrier layer that suppresses dendrite formation and a wetting layer that retains electrolytes. According to this document, it is desirable that the separator has a structure with small pores, and it is preferable that the separator has a laminate structure with two or more layers. Such structures provide a tortuous path for zinc dendrites and thus suppress a short circuit caused by dendrites.
Meanwhile, recently, a layered double hydroxide (LDH) represented by the general formula M2+1-xM3+x(OH)2An−x/n.mH2O (wherein M2+ is a divalent cation, M3+ is a trivalent cation, and An− is an anion having a valency of n) is known as a hydroxide ion-conductive solid electrolyte, and it is proposed that a membrane of such a layered double hydroxide is used as an alkaline electrolyte membrane for a direct alcohol fuel battery (see, for example, Patent Document 4 (WO 2010/109670)).